This invention relates to a golf club shaft formed of a carbon fiber-reinforced plastic and to a process for the fabrication thereof.
Carbon fiber-reinforced plastic (CFRP) shafts are now increasingly used because they are lighter in weight and provide golf clubs which can hit a golf ball longer distance as compared with conventional steel club shafts. In general, CFRP shafts are produced by a sheet winding method in which a prepreg sheet containing carbon fibers dispersed in a matrix of a hardenable resin is wound around a mandrel or by a filament winding method in which a tow or roving of carbon fibers impregnated with a hardenable resin is wound around a mandrel.
A CFRP shaft produced by the sheet winding method shows satisfactory mechanical strengths but encounters a problem because the flexural rigidity varies at different positions around the circumference. Thus, even with the same shaft and the same club head, the resulting golf club assembly gives a player a swing feeling that varies with the relative position therebetween. This follows that it is necessary to assemble the head and the shaft with a specific relative position.
While a CFRP shaft fabricated by the filament winding method is free from the above problem, the mechanical strengths thereof are insufficient to support a relatively large club head made of a metal. The recent trend is toward the use of a golf club having a light CFRP shaft and a large metal head. Thus, a CFRP shaft obtained by the filament winding method fails to meet with the recent demand.
There are proposals to prepare CFRP shafts by a combination of the filament winding and sheet winding methods. JP-A-57-142271 discloses a process for the production of a CFRP shaft having an improved strength and torsional rigidity, which includes the steps of winding a prepreg sheet containing carbon fibers dispersed in a matrix of a hardenable resin around a mandrel to form an inner layer, then winding a roving of carbon fibers impregnated with a hardenable resin around the inner layer to form an outer layer, and then hardening the inner and outer layers together.
JP-A-2-111380 discloses a process for the preparation of a golf club shaft having a tip end and a grip end, which includes the steps of winding a prepreg sheet containing carbon fibers dispersed in a matrix of a hardenable resin around a mandrel to form an inner layer extending from the grip end and terminating at least 100 mm away from the tip end, then winding a roving of carbon fibers impregnated with a hardenable resin around the inner layer to form an intermediate layer extending from the grip end to the tip end, then winding a roving of glass fibers impregnated with a hardenable resin around the intermediate layer throughout the length thereof to form an outer layer, and then hardening the inner, intermediate and outer layer together.
These publications are silent with respect to the reduction of the weight of the shaft to 1.4 g/in or less and the minimization of variation of flexural rigidity around circumference. It has also been found that the shafts obtained by the above processes still lack the mechanical strengths required for use with a large metal club head.